In general, a main relief valve is included in a hydraulic system of a hydraulic construction machine such as an excavator or the like, so that when a load pressure of a hydraulic pump exceeds a predetermined pressure, the main relief valve allows a hydraulic fluid discharged from a hydraulic pump to be drained to a hydraulic tank, thereby preventing a damage of hydraulic components. In addition, the hydraulic system employs a hydraulic control method in which the load pressure is restricted so as not to exceed a preset specific horsepower or torque value to reduce the discharge flow rate of the hydraulic pump.
The main relief valve applied to such a hydraulic system allows the hydraulic fluid to be drained to the hydraulic tank before the discharge pressure of the hydraulic pump reaches a preset value of the relief valve as shown in FIG. 2. In this case, since the hydraulic pump continues to discharge the hydraulic fluid, fuel is wasted unnecessarily due to an undesired driving of the hydraulic pump.
Conventionally, the hydraulic system is constructed such that when the discharge pressure of the hydraulic pump is more than the preset value of the relief valve, the relief valve is turned on or off in order to reduce a loss of the flow rate of the hydraulic fluid as mentioned above. In other words, when discharge pressure of the hydraulic pump is more than the preset value of the relief valve, the discharge flow rate of the hydraulic pump is abruptly reduced, and thus a shock occurs in the hydraulic system. This results in a deterioration of a manipulation feeling felt by an operator when a work apparatus such as a boom or the like is driven by the operator.